Experimental Determination of the Turning Point of Thermal Energy Helium Atoms above a Cu(001) Surface

The effective height of CO islands above a Cu(001) surface has been determined to be 2.3 Å from six interference maxima in the energy dependence of a specularly scattered He beam. Calculations of the He-CO-Cu potential and a best fit of the data show that the He-Cu(001) potential, recently predicted by Chizmeshya and Zaremba, must be shifted outwards by $0.64\pm 0.2Å$. Since the position of this potential is based on the electron spillout predicted by the jellium model, its use in the case of Cu(001) must be re-examined.